For example, for the purpose of power saving, technique to reduce a peak luminance of a display device is well known. However, if the peak luminance is reduced, a dynamic range of the display device falls. Because, by reducing the peak luminance, a difference of luminance between dark area and bright area in the image is decreased.
In order to remove this defect, by calculating a gamma signal of entire image and a gamma signal of each local region therein, technique to convert the image using the gamma signals is disclosed. By this technique, even if the peak luminance of entire image is lowly set, the image in which gradation collapse and luminance irregularity of each local region are suppressed can be displayed. On the other hand, in a display device on which bright black phenomenon is increased by external light reflection, in addition to fall of the peak luminance, dynamic range is further fallen by bright black phenomenon of dark part. In this case, even if the conventional technique is applied, fall of contrast of dark part in the image cannot be avoided.